In the prior art, there are various water heating equipment or systems including fast gas water heaters, solar energy water heaters, heat pump water heaters, water storage gas water heaters, electric water heaters and large central heating water systems. The various water heating equipment or systems described above become hot water source of daily hot water. From the hot water source to the water supply terminal, there are inevitably different lengths of hot water pipes. Among them, the cross-sectional area of a hot water pipe with larger inner pipe diameter can reach 3 cm2, that is, the volume of the hot water pipe per meter can reach 0.3 liters. When the length of the hot water pipe reaches 20 m, the volume of the hot water pipe can reach 6 liters. When the water is not used for a long period of time, the water in the hot water pipe becomes cold water. Each time the water is used, the cold water must be completely emptied before hot water reaches. The hot water pipe emptying time may be quite long. Thus, conventional water heating systems may not be convenient, and may waste excessive amounts of water and heat in the process of waiting for hot water and regulating water temperature. Among the various water heating equipment or systems described above, the fast gas water heater differs from other water storage heating equipment or systems in that, each time the system restarts the water supply, initial water flow may not be less than 3 L/min, moreover heating time of about greater than ten (10) seconds may be needed for heating cold water to hot water. That is, water pipe emptying time, in addition to water heating time, is incurred before hot water can be provided. Such hot water systems may be inconvenient relative to other water storage heating equipment or systems.
Another technique for supplying hot water is adding in the hot water supply pipe a backwater pipe from the water supply terminal to the hot water source to form a recirculation loop, which uses a pump as a peripheral device to circulate the water flow in the water pipes, keeping the water flowing through the supply terminal at an elevated temperature all, or most of, the time. If water in the water pipes circulates for extended periods of time, the hot water waiting time may be reduced to nearly zero. Unfortunately, because the actual water use time is much shorter than the waiting time, if the water pump constantly runs, due to the high temperature of water circulated in the pipeline for a long time, a lot of heat energy is wasted. If the method for starting circulating the water in the pump several tens of seconds before each use of water is adopted, waste of the heat and water is reduced as compared with the method for long-term circulation, the waiting time for hot water is increased, causing user inconvenience as well.
On the basis of the above, there is also a solution to automatically adjust water temperature by adopting mechanical or electronic automatic thermostatic water valve at the water supply terminal to mix cold water and hot water to adjust the water temperature. For a hot water storage source, it can achieve the goal that the temperature of water at the water supply terminal is kept constant. However, for a system using a fast gas water heater as a hot water source, when the fast gas water heater is a non-thermostatic type, because the above-mentioned thermostatic water valve is to maintain the water at thermostatic by changing the proportions of the hot water and the cold water while heat load is unchanged in the usage of the non-thermostatic type fast gas water heater, in case that the temperature of water flowing from the thermostatic water valve is higher than a preset temperature, the water flow coming from the fast gas water heater becomes less, the water temperature becomes higher, but the cold water flowing into the thermostatic water valve does not necessarily become more due to the thermostatic water valve itself or the pipelines, causing the temperature of water coming from the thermostatic water valve is increased, and thus the effect to maintain the temperature of water coming from the thermostatic water valve constant may not be achieved. As for the thermostatic fast gas water heater, a thermostatic water valve at the water supply terminal becomes redundant. Even in a water storage hot water supply, a thermostatic water valve at the water supply terminal can only automatically adjust the temperature of water coming from the water heater after the hot water flows to the water supply terminal, and does not solve the problem of long hot water waiting time in the hot water supply system or the heat energy waste in hot water pipeline circulation.
There is also a solution to solve the problem of the hot water waiting of the hot water supply system by adopting a small fast electric water heater or small electric water storage heater at the water supply terminal. When the small fast electric water heater is used for the water supply terminal, no matter whether it is thermostatically controlled or not, at the beginning of water usage, before the hot water from the hot water source reaches the water supply terminal, heat is provided by the small fast electric water heater. The heat load of the electric water heater may be required to reach 10-20 kilowatts so that there is enough hot water before the hot water coming from the hot water source reaches its destination. Such a large electric supply power may be unacceptable to a household power supply facility. When a small electric water storage heater with a small water storage tank capacity (for example, only 5-6 liters) is disposed at the water supply terminal, because the temperature of the water in the water storage tank can be up to 75° C., the water in the water storage tank may be mixed with the unheated water (e.g., city water) into hot water with the temperature which is the same with the preset water outlet temperature for usage by the user. For example, the hot water provided by the hot water source via the water supply pipe directly enters the water inlet terminal of the small electronic water storage heater, pushing the hot water in the water storage tank to be mixed with the unheated water. The flow amount of the hot water provided by the hot water source via the water supply pipe is equal to the hot water flow amount of the small electronic water storage heater. At this time, the temperature of water in the water storage tank is high, so the flow amount of the water flowing from the water storage tank must be small. The flow amount of the water flowing from the water storage tank is small at initial of using the water, and as the temperature of water in the water storage tank decreases, the flow amount of the water flowing from the water storage tank become larger. When using the fast thermostatic gas water heater as the hot water source, it may not be started in the beginning of using the water due to small water flow amount. The following situation may happen regardless of whether a fast thermostatic gas water heater, or another form of hot water source, is adopted to provide hot water. And, after the cold water flowing from the hot water source to the inlet of the water storage tank enters the water storage tank causing the temperature of the water in the water storage tank to drop to near or below the temperature needed by the water supply terminal, and before the host water from the hot water source does not arrive, there is a first-cold-then-hot phenomenon for a period of time. Therefore, it is hard to achieve the goal of proper usage of the hot water in a common small electronic water storage heater at the water supply terminal. Experiments show that, when the inner volume of the water pipe from the hot water source to the small electric water storage heater is about 80% of the inner volume of the water storage tank of the small electric water storage heater, the cold water temperature is below 15° C., and the temperature rise of the water at the outlet needs to be above 30° C., a problem of hot water supply interruption will arise. The worst condition in water supply is that, the temperature of the unheated water (e.g., city water) is about 5° C. and the thermostatic of water in the outlet is 50° C. Of course a solution of increasing the inner volume of the water storage tank of the electronic water storage heater can be adopted, but the inner volume of the water storage tank must be above 20 L in theory. When a thermostatic water is disposed at the water outlet, an ideal hot water supply status can be reached where the water supply terminal can provide continuous thermostatic hot water with the temperature the same as the preset water outlet temperature. However, it is economically meaningless due to large volume and high cost of the electronic water storage heater at the water supply terminal.